The present invention generally relates to a multi-cavity velocity modulation tube, and more particularly, to a high-gain wideband velocity modulation tube operable in the K-band higher than 10 GHz.
In a multi-cavity velocity modulation tube, a staggered tuning system is employed in which respective cavities are tuned to different frequencies in the pass band of the tube for the purpose of improving the gain versus frequency characteristics. Heretofore, in high-gain wide-band velocity modulation tubes operable in a microwave band lower than 10 GHz, in order to mitigate the variation of the cavity impedance in accordance with frequency variation, the Q-value of the cavity is lowered, and simultaneously the electron beam perveance is selected at a large value in the vicinity of 2.0 .times. 10.sup.-.sup.6 A/V so that the gain may not be lowered by the reduction of the Q-value.
On the other hand, in the K-band higher than 10 GHz, since the plasma wavelength is elongated to lengthen an effective drift length because of the fact that it is difficult to manufacture an extremely small cavity, it is necessary to select the electron beam perveance at 1.0 .times. 10.sup.-.sup.6 A/V or less. In addition, in the K-band, since an exciter having a high output power is not available, velocity modulation handling a high power must be carried out at a high gain. However, in order to realize a high gain with a low perveance, it is necessary to improve absorption of incident power in the input cavity by lowering the Q-value of this cavity and also to enhance the Q-values of the intermediate cavity or cavities and the output cavity. But there is a difficulty in that if these steps are carried out, then wide-band characteristics cannot be obtained, and on the contrary if the Q-values of the intermediate cavity or cavities and the output cavity were to be lowered to obtain wide-band characteristics, then the gain would be lowered.
According to one solution for such difficulty in a klystron amplifier to be used in the K-band as disclosed in U.S. Pat. No. 3,622,834, an electron beam perveance in a velocity modulation tube having a center band frequency of 12.2 GHz is selected at 0.5 .times. 10.sup.-.sup.6 A/V, an input cavity is tuned to a frequency slightly lower than the center band frequency, a first prebuncher cavity is tuned to a frequency lower than the upper band edge, a second prebuncher cavity is tuned to a frequency lower than the lower band edge, two buncher cavities are tuned to a frequency higher than the first prebuncher cavity, external loads are applied to the input cavity as well as the first and second prebuncher cavities, and the two buncher cavities are kept unloaded, whereby a small-signal gain of 59 dB can be realized when a .sup.-1 dB bandwidth is 32 MHz.
However, the velocity modulation tube constructed as described above has disadvantages that since the Q-value of the input cavity cannot be made extremely low because of the fact that the resonant frequency of the input cavity is selected to lie within the pass band of the tube, and since the effective Q-value of the buncher cavity is too low because the external load is supplied thereto despite the fact that debunching would occur in the second prebuncher cavity which is tuned to the frequency lower than the lower band edge, a sufficiently high gain and a sufficiently large bandwidth cannot be obtained; and that since hermetically sealed waveguides for connection to external circuits are mounted to the second prebuncher cavity in addition to the input cavity, the first prebuncher cavity and output cavity, the tube is accordingly of higher cost and also is inconvenient to handle.